1. Field of the Invention
The invention pertains to the field of chemical analysis of opaque slurries, such as chemical mechanical planarization (“CMP”) slurries. More specifically, it relates to an optical method of determining the chemical constituents of a CMP slurry.
2. Statement of the Problem
Chemical mechanical planarization (CMP) processes are used to planarize and smooth such surfaces in the integrated circuit and optics industries. CMP has emerged as the preferred method of planarization for manufacture of multiple layer semiconductor wafers having feature sizes less than or equal to 0.35 microns.
CMP is accomplished with a CMP polishing slurry and a complex polishing machine. Since the CMP process usually occurs in many different places in the integrated circuit or optics manufacturing line, generally the CMP slurry is continually pumped around a global distribution loop in the fabrication facility by a CMP slurry dispense module. Each polishing machine taps off the global loop.
CMP process slurries typically consist of a high concentration of sub-micron size abrasive particles, such SiO2, Al2O3, or CeO2 particles, suspended in a multicomponent liquid solution. The abrasive concentration is generally 4% to 18% solids by weight. SiO2 slurries are referred to in the art as “oxide” slurries, and Al2O3 slurries are referred to as “metal” slurries. CMP slurry manufacturers attempt to produce slurries that consist predominantly of particles less than 1.0 micron or even 0.5 microns in size.
The multicomponent solution usually includes one or more chemically active agents, such as an oxidizer or an acid or base solution. Since CMP slurries intimately contact the materials out of which the integrated circuit or optics components are made, to achieve repeatable, consistent results, it is important that the chemical content of the slurry be known and controlled. For example, materials that are planarized and chemicals used in integrated circuit manufacturing processes can get into the slurry and affect subsequent layers when they are planarized. Therefore, it is important to be able to periodically check the chemical composition of a CMP slurry. For chemical components that are stable over time, occasional monitoring of the chemical components may be sufficient. However, some chemicals and materials that make up the slurry can decompose over time. For example, most CMP slurries contain an oxidizer, the most common of which is hydrogen peroxide (H2O2). Hydrogen peroxide is currently included in 80% of all CMP slurries. H2O2 is unstable and decays over time. It is widely accepted that the H2O2 concentration in a CMP slurry must be continuously monitored in the global loop. As another example, benzotriazole (BTA) is a copper corrosion inhibitor commonly used in copper CMP slurries. If the concentration of BTA falls outside certain control limits, defects will result. In the past, monitoring of such chemicals as H2O2 and BTA has been done by analytical chemical means, such as titration. However, such methods are slow, consume approximately ten thousand dollars worth of reagent chemicals each year, and create a hazardous chemical waste stream that requires environmental treatment. Automated titration systems may require thirty minutes to make a measurement. In addition there is a high maintenance requirement due to the exposure of valves and related fluidic components in the titration system to the abrasive slurry.
Recently, the present inventor created a spectrometer system and method for measuring the chemical constituents of a CMP slurry. See United States Patent Publication No. 2003-0032366. However, this system works best in regions of the electromagnetic spectrum in which the scattering and absorption by the slurry is low to moderate. Hydrogen peroxide does not exhibit any significant optical absorption in the visible or near infrared spectral regions in which the above-mentioned system is most effective in the most common slurries. Hydrogen peroxide does exhibit a strong ultraviolet (UV) absorption band at wavelengths less than 300 nanometers (nm). In this spectral region, the abrasive components of common CMP slurries provide little or no optical transmission, such that previously described spectroscopic methods become impractical or impossible. Thus, the above-mentioned spectroscopic system and method is less than ideal for measurement of hydrogen peroxide, particularly when high accuracy is required.
Thus, there remains a need for a system and method for essentially continuous measurement of hydrogen peroxide and other slurry chemical constituents having absorption bands in spectral regions that coincide with very low optical transmission spectral regions of the other slurry components.